In recent years, water absorbent resins are widely utilized as a water absorbing agent in the sanitary materials such as paper diapers, sanitary napkins, and napkins for incontinents which is a constituent material thereof from the viewpoint of absorption of the body fluids. As such water absorbent resins, for example, a crosslinked product of partially neutralized polyacrylic acid, a hydrolysate of starch-acrylic acid graft polymer, a saponified product of vinyl acetate-acrylic acid ester copolymer, a hydrolysate or a crosslinked product of an acrylonitrile copolymer or an acrylamide copolymer, and a crosslinked product of a cationic monomer are known.
This water absorbent resin is generally used in the water absorbing agent as a powder (particles) although it can be used as a sheet, a fiber, and a film. As the powder (particles), for example, a particulate water absorbing agent having a mass average particle size of about from 200 to 800 μm is widely used. The particulate water absorbing agent is produced through many steps (preferably a continuous process) (for example, Patent Literature 1). In addition, the particulate water absorbing agent is filled in a filling container in the case of shipping this particulate water absorbing agent. The particulate water absorbing agent to be filled is transported by various means of transport to be delivered to the destination (user and the like). As this filling container, for example, a flexible container bag, a portable silo, and the like are used. This flexible container bag is also referred to as a fleconbag for short. Filling is usually conducted using a hopper as described in Patent Literature 2 or the like.
Meanwhile, a particulate water absorbing agent is produced by controlling a number of physical properties (for example, water absorption capacity, water absorption capacity under load, water absorbent speed, liquid permeability, and gel stability) of the parameters as the specifications depending on the intended use (for example, paper diapers and sanitary napkins). Various solving means as to be described below have been proposed in order to solve the problem of stabilization of such physical properties.
For example, Patent Literature 3 discloses a technique to remove the water absorbent resin having the respective physical properties which deviate from the predetermined upper and lower limits and to remix it. Patent Literature 4 discloses a technique to use plural hoppers in an intermediate step. Patent Literature 5 discloses a technique to conduct the polymerization in two systems and then to conduct a mixing treatment. As described above, a number of techniques for improving and stabilizing the physical properties of the particulate water absorbing agent by changing or imparting a new intermediate producing step have been proposed.
Furthermore, in recent years, a particulate water absorbing agent having a high water absorbent speed is desired, and in particular it is aimed to achieve both liquid permeability and water absorbent speed which are the physical properties conflicting with each other (Patent Literatures 6 to 12).
Meanwhile, there is not only a technique to precisely control the physical properties of the particulate water absorbing agent as described above but also a technique to eliminate a factor that causes a decrease in physical properties of the final product (for example, paper diapers) or consumer complaints by focusing on the fact that the deflection of the running conditions in the filling step to fill the produced particulate water absorbing agent causes a great deflection of the physical properties of the final product (for example, paper diapers) (Patent Literature 13).